As semiconductor dies are becoming very densely populated with active devices with a proportional increase in the number and density of interconnect layers, it becomes more difficult to perform diagnostic and failure analysis from the top or active side of the die because the density of the interconnects block optical and other signals. Therefore, backside analysis techniques are being developed, such as optical emission spectrophotometry, infrared analysis, focused ion beam milling and electron beam analysis to overcome the problem presented by the density of interconnect wiring. However many of these techniques require that most of the bottom or non-active side of the die be removed in a process called thinning. The resultant die thickness must be of the order of 15 to 100 microns, and uniform over chip sizes as large as 25 by 25 millimeters square.
Turning to the prior art, FIG. 1 shows a known prior art fixture comprising a top plate 1, connected to a bottom plate 2 by pins 3. Attached to bottom plate 2 are hard reference feet 4 numbering at least three and as many as six, dispersed in a ring along the periphery of the fixture and generally made from tungsten carbide which define a reference plane. A die plate 2A is axially located to which die 60 can be attached. Tapered rings 5 are used to set the exposed surface of the die into the reference plane defined by the reference feet. Spring 7 is used in combination with locking ring 6 to maintain the alignment of the sample. Knob 8 is used to gradually feed the workpiece into the abrasive media.
IBM technical disclosure bulletin Vol. 31, No. 4, pp. 402 to 404, describes a lapping/polishing fixture for microanalysis that utilizes micrometers to align a sample to a plane formed from three reference feet attached to the micrometers. The feet are made of nylon and are in contact with the polishing wheel during operation in order to maintain the planarity of the sample. The reference feet will quickly wear, destroying the reference plane when hard materials such as silicon are lapped.
U.S. Pat. No. 5,468,177 to Kindler et al. describes a method and apparatus for lapping air bearing surfaces on disk sliders. A compliant pad is used to press the workpiece into the abrasive media, while reference feet in contact with the abrasive media establish a reference plane. Because the workpiece is not rigidly held, corner rounding of large workpieces will occur.
A common feature of prior art fixtures is the requirement that the reference feet be in contact with the abrasive media at all times. When the reference feet are made of hard material, wear of the abrasive pad occurs and of the reference feet occurs over time. When the reference feet are made of soft material, the reference plane is quickly lost. In both cases, the result is a changing reference plane during the thinning process.